Chapter 26: Sound
Do You Hear What I Hear Sound produced by vibrations of material objects How is a sound wave produced in a: Violin? Saxophone? Flute? Your Voice?
Sound Travel Sound disturbances travel through mediums Sound Properties Pitch describes frequency of sound Pitches below 20 Hz = Infrasonic Pitches above 20000 Hz = Ultrasonic
Sound Properties As a source vibrates, a series of compressions and rarefactions (a disturbance in air where pressure is lowered) is produced.
Let’s Find a Happy Medium Sound can travel Faster in denser materials Solids > Liquids > Gases Sounds cannot travel in a vacuum no medium; nothing to compress and expand
Watch Your Speed Speed of sound Dependent on Slower than the speed of light (ex: you hear thunder after you see a flash of lightning) Speed of Sound at 0ºC = 330 m/s Dependent on Temperature Increases 0.60 m/s for every 1o C above 0 Elasticity of the medium
Keep the Noise Down Sound intensity can be measured proportional to square of the amplitude I α A2 Measured in Decibels Sound loudness subjective
You Can’t Force Me Forced vibrations are vibrations from one object that cause a nearby object to vibrate Examples: Music Box
It’s All Natural Natural frequency Frequency at which object will vibrate Frequency that requires least amount of energy to continue vibrations Depends on elasticity and shape of object
Isn’t It Resonating Resonance occurs when forced vibrations = Natural Frequency (increase in amplitude) Not restricted to wave motion
Resonance examples
Don’t Interfere Sound waves can be constructive or destructive Affects the loudness of sound Destructive Examples “Dead Spots” Antinoise technology ex: Jackhammer
Skip to the Beat Beats Caused by interference of two tones of slight frequency differences sounded together Freqbeat = IFreqA–FreqBI Humans can’t detect Beat frequencies over 7 Hertz What is the beat frequency when a 262 Hz and 266 Hz tuning fork are sounded together?
Closed Pipes Rules for pipe resonance: Node@closed end/Antinode@open end Smallest fraction of standing wave that fits is ¼ = Length Largest -> smallest frequency – called Fundamental frequency (f0) Other wave fractions that fit: ¾; (5/4); (7/4);… Frequencies for these are called harmonics H1 = 3f0; H2 = 5f0; H3 = 7f0; H4 = 9f0…
Open Pipes Same rules: Node@closed end/Antinode@open end Smallest fraction of standing wave that fits is ½ = Length Largest -> smallest frequency – called Fundamental frequency (f0) Other wave fractions that fit: ; (3/2); 2; (5/2)… Frequencies for these are called harmonics H1 = 2f0; H2 = 3f0; H3 = 4f0; H4 = 5f0…